1. Field of the Invention
The present invention relates to improved microfine propylene polymer powders which are spherical or substantially spherical in shape. The improved powders of the invention have reduced fines and a particle size distribution which renders them useful for powder coatings and especially for electrostatic spray coatings. The invention also relates to a process for producing the improved propylene polymer powders.
2. Description of the Prior Art
Thermoplastic resin powders are widely used in industry and these uses are well documented in the prior art. For example, powdered thermoplastic resins in dry form have been used to coat articles by dip coating in either a static or fluidized bed and by powder coating wherein the powder is applied by flame or electrostatic spraying or dusting. Powders can also be applied in dispersed form, by roller coating, spray coating, slush coating, and dip coating substrates such as metal, paper, paperboard, and the like. Powders are also widely employed for conventional powder molding processes, e.g., rotational molding and rotational lining. Still other applications for powders include use as paper pulp additives; mold release agents; additives for waxes, paints, caulks, and polishes; binders for non-woven fabrics; etc.
Besides the physical properties of the powder, which are dictated primarily by the resin being used, the size and shape of the powder particles are also important considerations in the selection of a powder for a particular application. These latter properties are primarily a function of the process by which the powders are prepared, which can include mechanical grinding, solution processes or dispersion processes. Particle size is determined using U.S. Standard Sieves or light scattering techniques and, depending on the method used, will be reported in mesh size or microns. The inverse relationship between the sieve size (mesh number) and particle size (in microns) is well documented and conversion tables are available. The shape of the particles is ascertained from photomicrographs of the powders. Particle shape has a marked influence on the bulk density and handling characteristics of the powder.
It is known that the presence of substantial amounts of "fines," i.e., particles having average diameters of 10 microns or less, can create problems in electrostatic powder coating operations. Some of the most common operational problems arising from the presence excess fines are identified by D. S. Tyler, Product Finishing, September 1990, pp. 23-26. Most significantly, the article points out that these fine particles have so little mass that they tend to be carried away from the product to be coated and into the reclaim system. The author cites studies which show that virtually no particles having an average particle size of less than 10 microns are retained on the coated part and concludes that substantial reduction or elimination of such particles is desirable.
Crystalline propylene polymers posses a desirable balance of toughness and chemical and solvent resistance. Furthermore, their low cost, low specific gravity, and low melt point make them useful for fusion coating processes. The utility of propylene polymers has been somewhat limited, however, due to the lack of availability of powders having suitable particle size and/or particle size distribution.
Powders can be produced using dispersion techniques, such as those described in U.S. Pat. Nos. 3,422,049 and 3,746,681. Such processes produce particles which are spherical in shape and have a relatively narrow size range, i.e., particle size distribution. These dispersion procedures involve subjecting the molten resin in about 0.8 to 9 parts by weight of water per part of resin to vigorous agitation in the presence of from about 2 to 25 parts by weight per 100 parts of resin of a water-soluble block copolymer of ethylene oxide and propylene oxide having a molecular weight above about 3500 and containing at least about 50% by weight of ethylene oxide so that a fine dispersion is produced. The resulting dispersion is then cooled to below the softening temperature of the resin and the powder recovered. A continuous dispersion process for the preparation of finely divided polymer particles is disclosed in U.S. Pat. No. 3,432,483. The process comprises the sequential steps of feeding to the polymer, water and a water-soluble block copolymer of ethylene oxide and propylene oxide surfactant into a dispersion zone; vigorously agitating the mixture under elevated temperature and pressure to form a dispersion of the molten polymer; withdrawing a portion of the dispersion and cooling to a temperature below the melting point of said polymer to form solid, finely divided polymer particles in the dispersion; reducing the pressure of said cooled dispersion to atmospheric pressure; separating the solid polymer particles from the surfactant solution phase and washing; drying the washed polymer particles; and recovering dry, finely divided powder.
While microfine powders of propylene polymers can be produced using the above-described dispersion processes, the powders obtained typically have a large number of particles which are 10 microns or less in size. The presence of these fines not only limits the utility of the product but also presents recovery problems. It would be highly desirable if microfine powders of crystalline propylene polymers could be produced wherein the amount of fines is significantly reduced. These and other advantages are realized by the present invention which is described in more detail to follow.